Our subjective impression of the visual world is that everything is in focus. This impression is reinforced by the common practice in photography and cinematography of creating images that are in focus everywhere. Our subjective impression, however, is quite incorrect because most parts of the retinal image are significantly blurred. We will examine the use of blur and the eye's focusing response in the perception of distance and size. We will also examine how blur and accommodation affect viewer fatigue. There is clear evidence that blur affects distance and size perception. A probabilistic model will be developed that incorporates the information in blur and other cues. The model will be evaluated and refined by comparing its behavior to human perception in a series of psychophysical experiments. One set of experiments will use miniaturization effects developed in photography. Those experiments will determine the contributions of blur and the eye's focusing response (accommodation) on perceived absolute distance. Another set of experiments will capitalize on the information contained in the blur of a border between two regions in an image. Those experiments will determine whether the plausibility of observed blur, focus distance, and relative size affect the perception of depth order. The geometries of depth from binocular disparity and depth from blur are fundamentally similar. In principle, the two cues provide complimentary information about distance, with disparity providing more precise information near the point of fixation and blur providing more precise information away from fixation. The relative contributions of disparity and blur to perceived distance and eye-movement control will be measured for various positions relative to where the viewer is looking. The amount of blur in the retinal image depends on the absolute and relative distance of objects in the environment and on where the viewer is focused. We define the concept of natural depth of field; this is the appropriate relationship between other depth cues and blur for a normal human eye viewing the natural environment. We will determine whether people are sensitive to natural depth of field and whether 3D structure is misperceived in images that deviate from the natural relationship. Stereo displays are becoming more widely used in applications such medical imaging, surgical training, and scientific visualization. The motion picture and television industries are also rapidly introducing stereo. With stereo displays, the normal correlation between vergence and accommodation is disrupted. The result is a vergence-accommodation conflict. There are several adverse consequences of the conflict, but the most important is undoubtedly discomfort and fatigue. We will determine how viewing distance, refractive error, age, and the magnitude and sign of the vergence-accommodation conflict affect discomfort and fatigue. A better understanding will aid the design of stereo displays and the content shown on such displays.